1. Field of the Invention
This invention relates generally to voltage/current driver/regulator circuit design and, more particularly, to the design of a regulator circuit operating with multiple supply voltages.
2. Description of the Related Art
A variety of driver circuits exist today for use in integrated circuits and systems for driving a signal line or a bus. Oftentimes driver circuits are configured to enable bus transactions between a source device and a target device, and feature complex designs in order to meet various system specifications. Typically, these driver circuits may be relatively expensive to build. Other driver circuits may feature simple or simpler designs, failing, however, to accurately control output currents and output voltages, while also having slow rise and fall times.
One variety of driver circuits generally comprises a relatively low power circuit that drives, or controls, a higher power device, which may be part of a power driving stage for a load. One example might be a load that is a motor, such as a brushless motor, that provides the motive force for a fan. Fans are oftentimes used in computer systems to evacuate hot air from enclosures to prevent certain circuit components, such as central processing units (CPUs) from overheating. Linear driver circuits are therefore often used to drive the fan motor, and/or controlling the rotational speed of the fan in a wide variety of computer systems.
Linear drivers are also a feature of output stages in many amplifiers—whose basic function is to produce an output signal with a power that is a multiple of the power of an input signal—since many applications call for an output waveform that faithfully reproduces the shape of the input signal while magnifying its voltage and/or current in a linear fashion. In order to increase the efficiency of an amplifier while maintaining a high degree of linearity, a class G design may be employed, which involves changing the power supply voltage from a lower level to a higher level when larger output swings are required.
A variety of methods and solutions exist for implementing class G operation in an efficient manner. The simplest solution typically involves a single class AB output stage connected to two power supply rails by a diode, or a transistor switch. In this solution, under most circumstances, the output stage is connected to the lower supply voltage, and automatically switches to the higher rails for large signal peaks. Another approach features two class AB output stages, each stage connected to a different power supply voltage, with the signal path determined by the magnitude of the input signal. Using two power supplies improves power efficiency enough to allow significantly more power for a given size and weight.
Class G amplifiers typically include current blocking diodes configured to prevent driving current into a lower voltage supply when the amplifier output exceeds the lower supply voltage. While this provides effective protection, it also places a limit on the efficiency of the contribution provided by the lower voltage power supplies to the overall amplifier output. Some power will unavoidably be dissipated in the diode as a result of the voltage drop across the diode, any time a lower voltage supply is contributing to the overall output of the amplifier. In addition, a power device typically included in each output stage to control the flow of current to the load will dissipate power that is equal to the load current multiplied by the difference between the supply voltage and the amplifier output. This power would be wasted any time the supply contributed to the amplifier output.
When a power supply is contributing maximum current to the amplifier output, the output device may be operating in either saturation mode or linear mode, generally with a voltage drop in the tenth volt range (typically few tenths of a volt). When the voltage drop across the output device is combined with the voltage drop across the diode when using a lower voltage supply, the total difference between the supply voltage and the amplifier output may be around one volt. While such a voltage drop and corresponding inefficiency may be acceptable in relatively high voltage amplifiers where the output is in the 10V range (typically tens of volts), integrated circuit amplifiers for low-power applications are typically designed to operate with minimum supply voltages below two volts, and such a drop in output stage voltage would limit the amplifier's maximum efficiency to less than fifty percent.
One solution for the design of more efficient class G amplifiers is described in U.S. Pat. No. 6,838,942 (Efficient class-G amplifier with wide output voltage swing). According to this solution, the amplifiers include multiple output stages, each associated with a distinct supply voltage, the amplifiers thereby operating off of multiple supply voltages. Each output stage contributes current to the output of the amplifier over a range of amplifier output voltages, with possibly overlapping voltage ranges. Each output stage also contributes current until the amplifier output voltage reaches the supply voltage associated with that output stage. When the amplifier output voltage is close to the supply voltage associated with an output stage, both that output stage and the output stage associated with the next highest supply voltage may contribute to the amplifier output.
Certain drawbacks of this solution are apparent, however. For example, current may flow from a high supply to low supply during fast signal transients, causing extra power dissipation. This may occur due to the pass devices to the two supplies conducting simultaneously, in addition to the slow speed of the control loop. Another issue may be the less than optimal power saving due to a transition region from one supply to the next supply having a value in the 100 mV range (typically hundreds of mV). Finally, the feedback loop can be very difficult to stabilize.
Other corresponding issues related to the prior art will become apparent to one skilled in the art after comparing such prior art with the present invention as described herein.